It's called spatula striping...just take a spatula and make a stripe of pink (for example) from the tip of the icing bag all the way to the top of the bag. Then fill with white frosting. Then do your decorating... Hope this helps...

I don't think this is spatula striping. I do that all the time. This looks to be white on one side of the bag and pink tinted icing on the other side of the bag. When you squeeze down, then they will merge like you want. Hope that helps.